This invention relates generally to instrumentation for making viscoelastic measurements and, more particularly, to an improved vibratory viscoelastic transducer for measuring the viscoelastic characteristics of fluids and gels.
Vibratory viscosity and viscoelastic analyzers are well known in the prior art. These analyzers typically incorporate a mechanical probe member that is driven to move in an oscillating or periodic manner. They include some means for imparting motion to the probe member and means for monitoring the probe displacement or motion. Most designs incorporate electronic means for driving the mechanical probe and monitoring the probe motion. The moving probe is immersed within a fluid or gel being tested. The motion characteristics of the moving probe are affected by the viscous and viscoelastic properties of the material being tested. The change in motion characteristics of the probe in response to a test sample is monitored and processed by the analyzer to determine the viscoelastic properties of the test sample.
The components of the drive mechanism or circuit, the mechanical probe, and the motion detection mechanism collectively comprise a transducer. The transducer has response characteristics that relate the input drive signal to the output motion signal. The response of the transducer is dependent on its physical design and the viscoelastic properties of the material being tested. The transducer characteristics can be represented using mechanical or electrical performance properties. U.S. Pat. No. 4,341,111 to Husar employs mechanical displacement versus frequency to characterize transducer behavior. This type of characterization is illustrated in FIG. 6. From the perspective of the electrical design, it is often desirable to characterize the transducer behavior in terms of electrical impedance. FIG. 5 illustrates the change in electrical impedance versus frequency for a viscoelastic transducer like that shown in FIGS. 1 and 2. This type of transducer monitors probe velocity rather than probe displacement. Both displacement and velocity monitoring transducers are nonlinear with respect to drive frequency and typically exhibit a pronounced peak at the natural resonant frequency. This resonant frequency varies in response to the viscoelastic properties of the liquid or gel being measured. These peaks are present in the examples illustrated in FIGS. 5 and 6.
Viscoelastic transducers vary widely in their design. One variation relates to the direction of probe motion. Prior art designs have incorporated axial, lateral, orbital or radial probe motion within the transducer. Exemplary of prior art transducers that employ an axially vibrating probe are those described in U.S. Pat. Nos. 3,587,295 and 3,741,002 to Simons, as well as U.S. Pat. No. 4,026,671 to Simons et al. U.S. Pat. No. 4,312,217 to Hartert and U.S. Pat. No. 4,341,111 to Husar teach the use of an elastic rod driven to produce either lateral or orbital motion. U.S. Pat. No. 4,488,427 to Matusik et al. teaches rotational or radial mechanical oscillation. These prior art transducers incorporate a single flexible member that deforms to produce the desired mechanical motion. The axial vibrating transducer taught by the Simons and Simons et al. patents incorporates a diaphragm. The lateral or orbital motion taught by Hartert and Husar is achieved by employing an elastic rod driven so as to bend the rod in one direction for lateral motion or in two directions for orbital motion. The radial motion taught by Mutasik et al. is produced by employing an elastic cylindrical tube driven with a torsion force producing radial deflection.
Most vibratory transducers employ separate drive and pickup coils. However, the separate coils are not required. The transducer taught by Simons et al. incorporates a single coil used simultaneously for driving the transducer and for monitoring mechanical motion.
Viscosity or viscoelastic analyzers monitor the transducer response using suitable circuitry and possibly computational techniques to determine the viscous or viscoelastic properties of the material being tested. The circuitry for driving vibratory transducers is well known in the art and typically consist of a driving circuit to produce the oscillating mechanical motion and a monitoring circuit to monitor the mechanical motion. Additional circuitry, including means to regulate mechanical motion, monitor energy consumption, or condition signals for display is also taught in the prior art. Early designs drive the transducer at fixed frequencies. Later designs typically drive the tranducer at the resonant frequency of the transducer immersed within the material being tested. In the later devices, the viscoelastic properties of the material under test are determined by analyzing only the resonant point of the vibrating transducer. The attenuation of the resonant peak characterizes the viscous property of the test sample, while the change in frequency characterizes its elastic property.
The accuracy of viscosity and viscoelastic measurement devices incorporating vibratory transducers is limited by the performance characteristics of the vibratory transducer. Prior art vibratory transducers are prone to poor manufacturing reproducibilty, poor oscillation characteristics, temperature drift, long term drift, and humidity drift. Additionally, the durability of these transducers is lacking. Damage from operator handling can cause either transducer drift or failure. In some designs, the transducer moving coil can be pushed against a magnet or magnet pole, thereby potentially damaging the coil. In other designs, the transducer alignment can be compromised by minor operator mishandling, thereby causing transducer drift or impeding transducer oscillation.
None of the prior art transducers have specificaly addressed the need for protecting the oscillating mechanical component from excessive applied force. While the transducer of Simons oscillated axially, the operator could cause lateral or excessive axial movement of the diapragm while mounting or removing a disposable probe. This excessive movement could result in the coil contacting either the magnet or magnet pole and thus damaging the coil. Approximately 95% of all service requirements for this design have involved damaged coils. Additionally, the transducer characteristics were prone to change if the diaphragm was stressed by excessive axial movement.
Summarizing the prior art, the oscillating transducer taught in U.S. Pat. No. 4,341,111 to Husar relys on lateral or orbital movement of an elastic rod. However, this reference does not address protecting the elastic rod from excessive deformation. The transducer taught in U.S. Pat. No. 4,488,427 to Matusik et al. involves a tube that is subjected to rotational deflection and in which alignment of the transducer requires that the tube the prevented from deflecting laterally. However, this reference does not address protecting the rotational tube from undesired lateral deflection. Similarly, the oscillating transducers taught in U.S. Pat. Nos. 4,154,093 and 4,869,098 have no provision for mechanical protection against excessive transducer displacement.
It is therefore a principal object of the present invention to provide a vibratory transducer that minimizes operator handling damage by providing mechanical stops that limit the elastic deformation of the mechanical probe and embody a mechanical probe design that is substantially rigid except in the desired elastic direction of motion.
This and other objects are accomplished in accordance with the illustrated preferred embodiment of the present invention by providing a circular spring assembly coaxially mounted to a mechanical probe to restrict motion of the probe member in all directions except the desired direction along the axis of the probe and by providing mechanical stops for limiting the range of desired motion along the axis of the probe.